Imaging central nicotinic acetylcholine receptors in baboons with [18F]fluoro-A-85380

Central nicotinic acetylcholine receptors (nAChRs) have been implicated in learning-memory processes. Postmortem brain tissue of patients who suffered senile dementia or Parkinson's disease shows low density of nAChRs. In this study, we used PET to evaluate the distribution and kinetics of the fluoro derivative of the high-affinity and alpha4beta2-subtype-selective, nicotinic ligand 3-[2(S)-2-azetidinylmethoxy]pyridine (A-85380) in baboons.

After intravenous injection of 37 MBq (1 mCi, 1-1.5 nmol) [18F]fluoro-A-85380 into isoflurane-anesthetized baboons, dynamic PET data were acquired for 180 min. Time-activity curves were generated from regions of interest. Displacement experiments (80 min after injection of the radiotracer) were performed using cytisine (1 mg/kg subcutaneously) and unlabeled fluoro-A-85380 (0.1 and 0.3 mg/kg intravenously). Toxicological studies were performed in mice.

Brain radioactivity reached a plateau within 40-50 min of injection of the tracer. In the thalamic area, radioactivity remained constant for 180 min, while clearance from the cerebellum was slow (t1/2 = 145-190 min). Cytisine and unlabeled fluoro-A-85380 reduced brain radioactivity at 180 min by 50%-60%, 30%-35% and 20%-35% of control values in the thalamus, cerebellum and frontal cortex, respectively. A slight, transient increase (20 mm Hg) in blood pressure was observed with the highest displacing dose of unlabeled fluoro-A-85380. Lethal dose in mice was found to be 2.2 mg/kg intravenously.

These results demonstrate the feasibility and the safety of imaging nAChRs in vivo using labeled or unlabeled fluoro-A-85380.